1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to an apparatus and a method for detecting an antenna mispointing condition.
2. Description of the Related Art
Antennas used at satellite Earth Stations (ES) that provide Fixed Satellite Services (FSS) are aligned with high accuracy (.+-.0.5 degree) to point at Geosynchronous Orbit (GSO) satellites located at a specific locations in the sky. Presently, there are three approaches that are used for detecting antenna mispointing conditions. First, ES performance is actively monitored using trained technicians. Second, a steerable ES antenna is used that is controlled using well-known closed-loop techniques for optimizing link performance. Third, downlinked FSS signals are monitored for confirming reception of known network messages and the network responses to transmissions from the ES.
The present approaches for detecting antenna mispointing conditions are satisfactory for current frequency bands of operation and for limited numbers of earth stations. However, newer FSS networks will provide ubiquitous services by using large numbers of unsupervised earth stations. Since the unsupervised earth stations are contemplated to be inexpensive consumer-type electronics, actively-controlled (steerable) antennas are generally not being considered. Consequently, the first and second antenna mispointing detection approaches are not practical. The third approach currently used is the only approach that would be applicable for detecting antenna mispointing conditions for the unsupervised earth stations.
Many of the new FSS networks that will use the inexpensive earth stations will operate in new frequency bands in which considerable weather impairments potentially exist, such as signal attenuation caused by rain and other adverse atmospheric conditions. To counter the weather-related impairments, the unsupervised earth stations will employ open- and closed-loop power control features. That is, ES transmissions will be monitored by the network. When the signal strength of transmissions from an ES are weakened by an adverse atmospheric condition, the network will issue a command to the ES for an increase in transmit power using open-loop techniques at the ES. When transmissions are so weak that link error rate(s) increase or no network acknowledgements are received by an ES, the ES and/or satellite will continue to gradually increase transmit power until the link errors are reduced or acknowledgements are received and closed-loop power control techniques can be performed. If, at the highest permissible ES transmit power level, the network still does not respond with an acknowledgement, the ES determines that there is an error condition and transmissions will cease.
FIG. 1 shows a schematic block diagram of a conventional RF antenna subsystem 10 for an earth station. A demodulator 11 demodulates received RF signals in a well-known manner to produce a data-in signal. An earth station system controller 12 is coupled to demodulator 11 by a power monitor signal that is output from demodulator 11. A demodulator control signal that is output from controller 12 is coupled to demodulator 11. Subsystem 10 also includes a modulator 13 that modulates data for RF transmission in a well-known manner. Modulator 13 is coupled to controller 12 through a modulation control signal that is output from controller 12. The output of modulator 13 passes through a gain control device 14 before being coupled to an antenna (not shown). Gain control device 14 operates in a well-known manner and is controlled by a Tx level control signal that is output from controller 12.
FIG. 2 shows a flow diagram of a conventional process 20 for establishing satellite communications links for the conventional earth station antenna subsystem 10 that is shown in FIG. 1. If, at step 21, no downlink signal is detected, then an error condition is indicated in a well-known manner by the subsystem at step 22. Otherwise, open-loop power control is performed for optimizing the transmit power required for completing the radio communications path between the earth station and another radio communications device. At step 23, a service request is transmitted by the earth station. At step 24, controller 12 determines whether the network has responded to the service request. If not, flow continues to step 25 where controller 12 controls gain control device 14 to increase the output transmit power of the earth station. At step 26, it is determined whether the maximum transmit power has been exceeded. If so, flow continues to step 27 where an error is indicated in a well-known manner by the subsystem because the network has not responded and the maximum transmit power has been exceeded. If the maximum transmit power has not been exceeded, flow continues back to step 24. When a network response is detected before the maximum transmit power is exceeded, flow continues to step 28 where well-known closed-loop power control and synchronization processes are performed by the subsystem.
A problem associated with this conventional approach for detecting antenna mispointing conditions is that in some frequency bands, the acceptable attenuation from weather impairments exceeds the acceptable attenuation caused by antenna mispointing. Consequently, it is possible for a mispointed ES antenna to remain operational in clear atmospheric conditions. That is, the FSS network would simply treat signal attenuation caused by antenna mispointing as a weather impairment and issue commands to the ES for increasing the ES transmit power above the nominal link budget. However, a mispointed ES antenna can potentially cause interference levels to be increased at an adjacent satellite since the mispointed ES will be operating at a large transmission power margin that would normally occur only in adverse atmospheric conditions.
Therefore, there is an need for reliably differentiating between received signal impairments caused by adverse atmospheric conditions and by antenna mispointing so that appropriate countermeasures can be performed. For example, power control can be used for weather-related impairments while an operator error signal can be used for indicating a mispointed antenna.